Ni-Coated Ti Powders

ABSTRACT

The present invention relates to coated powder, comprising a Ti-bearing core and a Ni-bearing coating, which can be used for the production of porous Ni—Ti articles by the self-propagating high temperature synthesis (SHS) method. The obtained articles are ideally suited for use in biomedical applications. According to the invention, a coated powder is used comprising a metallic Ti-bearing core and a metallic Ni-bearing coating, characterised by a Ni:Ti atomic ratio of more than 0.5, preferably between 0.9 and 1.1, and more preferably between 0.96 and 1.04. By using coated powders, local fluctuations in composition are limited and well under control. Milling of powders and the ensuing contamination risks are avoided. The sintered objects obtained using coated powders have a more homogeneous porosity than that using mixed Ni and Ti powders.

The present invention relates to coated powder, comprising a Ti-bearingcore and a Ni-bearing coating, which can be used for the production ofporous Ni—Ti alloys.

Such a porous Ni—Ti alloy article is described in U.S. Pat. No.5,986,169. The article has a porosity of 8 to 90% and is defined by anetwork of interconnected passageways extending throughout. The networkexhibits an isotropic permeability permitting complete migration offluids. The material is elastically deformable. These characteristicsrender it useful in biomedical and other applications. For producing theporous article, the so-called self-propagating high-temperaturesynthesis (SHS) method is used in which the alloy is produced by meansof a layered combustion which exploits exothermic heat emitted duringinteraction of elemental nickel and titanium powders.

U.S. Pat. No. 2,853,403 describes a method for producing composite metalpowders. This method consists of dispersing, in solid form, particles ofone or more metals of interest as nuclei in an ammoniated solution inwhich another metal of interest having different chemical and/orphysical properties is present as a soluble salt, and precipitating thelatter metal from the solution by gas reduction to form composite metalparticles in which the dispersed metal particles are coated by theprecipitated metal. This method was however never specifically appliedfor the manufacture of Ni-coated Ti powder.

The use of elemental Ni and Ti powders renders the production processsensitive to segregation problems, resulting in composition fluctuationsand/or non-homogeneous porosity. Composition fluctuations can in turnlead to the formation of unwanted secondary phases.

According to the invention, the above drawbacks can be overcome usingcoated powder, comprising a metallic Ti-bearing core and a metallicNi-bearing coating, characterised by a Ni:Ti atomic ratio of more than0.5, preferably between 0.9 and 1.1, and more preferably between 0.96and 1.04. Atomic ratios of more than 0.5 to about 1 are preferred asthis avoids the formation of secondary phases and yields an alloy withbetter mechanical properties.

The Ti-bearing core may consist of pure Ti, while the Ni-bearing coatingcan consist of pure Ni.

It may be useful to mix Ti-bearing powder or Ni-bearing powder with thecoated powder so as to obtain a Ni:Ti atomic ratio of the mixture ofbetween 0.9 and 1.1, and preferably between 0.99 and 1.01. Thisprocedure allows for easy adjustments to the Ni:Ti ratio.

The particle size of the powders should preferably be finer than 150mesh.

Above described powders can be used for the manufacture of sinteredobjects, possibly using the SHS technique.

Another object of the invention concerns a process of manufacturing acoated powder comprising the steps of:

-   -   providing for suitable quantities of a Ti-bearing powder and of        a Ni salt bearing aqueous solution;    -   feeding said powder and said solution in an autoclave together        with a quantity of NH₄OH, and, optionally, with a quantity of        ammonium salts;    -   precipitating the Ni onto the Ti-bearing powder by hydrogen        reduction;    -   washing, filtering and drying the slurry obtained, thereby        obtaining a Ni-coated Ti powder.

The Ni is preferably precipitated onto the Ti-bearing powder at atemperature of at least 100° C. and a hydrogen pressure in the autoclaveof at least 1.4 MPa.

Powders obtained by this coating process are, as such or after mixingwith Ni-bearing or Ti-bearing powders, suitable for SHS sintering ofobjects.

By using coated powders, local fluctuations in composition are limitedand well under control. Ni-coated Ti powder also decreases the diffusiondistance between the Ni and Ti atoms, which may eliminate or reduce theformation of unwanted intermetallic compounds such as Ni₃Ti and NiTi₂.It has been found that the porosity of the porous titanium-nickelproduced by SHS starting from Ni-coated Ti powder is more homogeneousthroughout the sample compared to starting with elemental powders. Ithas also been found that the compactability of Ni-coated Ti powder issignificantly better than that of elemental powders. Because of this,next to the possibility for a decreased die wear, larger devices can beproduced. Additional advantages are that no milling is required, thusavoiding contamination such as oxidation of Ti during the preparationstage, and that the intimate contact between Ni and Ti makes it possibleto perform SHS with no or less preheating compared to green compactsmade of elemental Ti and Ni powders.

The SHS process can produce Ni—Ti alloys with large pore volumes and athree-dimensional interconnected network of pores and channels. Thisporous network is particularly suitable for implants to achieve securetissue-to-implant bonding. Pre-alloyed powder, such as atomised Ni—Ti,does not work for the SHS process, since it is already an alloy and theexothermic reaction does not take place during sintering.

During the manufacturing the coated powder, 50 to 500 g/l ammoniumsalts, such as (NH₄)₂SO₄ or (NH₄)₂CO₃, may be added, to prevent theformation of unwanted Ni(OH)₂ and to ensures a smooth coating.

The following Figures illustrate the invention.

FIG. 1 shows an SEM of coated product.

FIG. 2 shows an EDS map of the cross section of Ni-coated Ti powder; thesolid Ti cores (left) and the Ni-coating (right) are visible.

FIG. 3 gives a longitudinal view of powder A after compaction and SHS;the arrow indicates the direction of the propagation front.

FIG. 4 is a SEM-image of powder A (left) and powder B (right) after SHS.

FIG. 5 shows an XRD spectrum of Ni-coated powder D after SHS.

FIG. 6 shows macroscopic pictures of samples made by the SHS processusing different raw materials: (a) using powder D, (b) powder E, (c)powder F, (d) powder G.

FIG. 7 shows SEM pictures of samples made by SHS using various rawmaterials at low (top) and high (bottom) magnification: (a) and (d) forpowder D, (b) and (e) for powder F, (c) and (f) for powder G.

PREFERRED EMBODIMENT 1

Ti powder along with a Ni bearing solution, such as a sulphate or acarbonate, and, in particular when a sulphate is used, ammoniumhydroxide (ammoniac), preferably in a NH₃:Ni ratio of 2:1, are fed to anautoclave. A surface-active additive, such as anthraquinone, is alsoadded to the solution to an amount of 0.2 to 5 wt. % of the Ti powder.This ensures a smooth coating of the Ti particles. The Ni is thenprecipitated on the titanium surface using H₂ at a temperature of 100 to200° C. and at an H₂ pressure of 1.4 to 3.4 MPa. After coating, theslurry is washed, filtered and dried.

EXAMPLE 1

The result of coating a batch of Ti powder as described in preferredembodiment 1 is given in Table 1. The reduction temperature was 150° C.and the reduction pressure was maintained at 3.4 MPa. A SEM (ScanningElectron Microscope) picture of the coated product is shown in FIG. 1.An EDS (Energy Dispersive Spectroscopy) map of the cross section of thepowder is shown in FIG. 2. SEM and EDS maps show a homogeneous andsmooth coating.

TABLE 1 Results of coating Feed Coated powder Ni (g/L) Ti (g/L) Ni wt. %Ti wt. % 21.8 21.6 53.2 46.6

EXAMPLE 2

Ni-coated Ti powder was produced starting from 3 types of Ti powderhaving a different particle size distribution:

-   -   powder A: Ni coated −400 mesh Ti powder;    -   powder B: Ni coated −250+325 mesh Ti powder;    -   powder C: Ni coated −150+200 mesh Ti powder.

The composition of the coated powder is shown in Table 2.

TABLE 2 Composition of the coated powder Powder Composition reference Niwt. % Ti wt. % A 53.8 45.8 B 53.7 46.1 C 53.3 46.0

EXAMPLE 3

The three different powders were die-compacted on an Instron-press to adensity of respectively 48%, 59% and 51% of the theoretical densityusing a compaction load of 22 kN, 19 kN and 11 kN respectively.

SHS performed on compacted powder A requires an ignition time of lessthan 10 seconds. The propagation front is parallel and stable and theresulting sample dimensions are also stable (FIG. 3). Powders B and Cshowed a tendency to more intensive melting in the upper part of thesample.

Two types of pores are present: small ones and large elongated onesperpendicular to the propagation front direction (FIG. 4). As theinitial particle size increases from powder A to powder C, the width ofthe elongated pores increases from roughly 200-300 μm to 400-600 μm andfinally to 800-1000 μm. The porosity distribution in each sample ishomogeneous, except in the regions where a large amount of liquid phasewas present, resulting in lower porosity.

The phases present in the SHS-product have been determined using XRD(X-Ray Diffraction) and EDX (Energy Dispersive X-ray) analysis. The XRDdiagram in FIG. 5 clearly shows the presence of the desired Ni—Ti phase,both monoclinic and cubic, and possibly a limited amount of NiTi₂.

EXAMPLE 4

To be able to compare Ni-coated Ti powder with elemental Ni and Tipowders, the following batches were prepared:

-   -   powder D: Ni coated −400 mesh Ti powder, blended with some        additional Ni powder;    -   powder E: Ni coated −250+325 mesh Ti powder, blended with some        additional Ni powder;    -   powder F: Ni coated −150+200 mesh Ti powder, blended with some        additional Ni powder;    -   powder G: Ni powder of 1.2 μm (d₅₀), mixed with −250+325 mesh Ti        powder in a 1:1 atomic ratio (55.07:44.93 Ni:Ti wt. % ratio).

Based on the composition analysis of the Ni-coated Ti powder, additionalfine Ni powder was blended with the coated powder to balance the Ni:Tiatomic ratio to 1:1. The addition of Ni powder is shown in Table 3.

TABLE 3 Amount of Ni added to 100 g of Ni-coated Ti powder Powderreference Ni:Ti (wt. %) Ni powder added (g) D 53.8:45.8 2.34 E 53.7:46.12.80 F 53.3:46.0 3.08

Quartz tubes with a diameter of 20 to 25 mm and a length of 130 to 170mm were used for containing the powder. Powder mixture G was ball milledfor 2 hours before being loosely packed in a quartz tube. Green densityof the mixed powder was about 50 to 60%. A load of 30 to 40 kN wasneeded to press the sample.

The green densities for powders D, E and F were respectively about 45%,50% and 65%, accomplished using loads of 10 kN, 15 kN and 18 kNrespectively.

All samples were placed in a vacuum chamber with a vacuum of about 0.01Pa. After pre-heating the samples to 350° C. for 1 hour, the sampleswere ignited. SHS took place.

FIG. 6 shows macroscopic pictures of the samples prepared by SHS. Thesurface morphology of the samples made by Ni-coated Ti powder washomogeneous. The surface morphology of the samples made by mixed Ni andTi powders was rough and the porosity was inhomogeneous.

SEM pictures in FIGS. 7( c) and 7(f) show that the pore size andmorphology of the sample made from mixed Ni and Ti powder areinhomogeneous. FIGS. 7( a), 7(b), 7(d), and 7(e) show that the pore sizeand morphology prepared from finer Ni-coated Ti powders are morehomogeneous than those by coarser Ni-coated Ti powder. There are alsomore open pores in the samples using finer Ni-coated Ti powders.Overall, samples using Ni-coated Ti powder have a more homogeneousporosity than that using mixed Ni and Ti powders.

1-12. (canceled)
 13. A process of manufacturing a Ni-coated Ti powder comprising a metallic Ti-bearing core and a metallic Ni-bearing coating, wherein the coated powder has a Ni:Ti atomic ratio of more than 0.5, the process comprising the steps of: (a) providing a Ti-bearing powder and a Ni salt bearing aqueous solution; (b) feeding the Ti-bearing powder and the Ni salt bearing solution into an autoclave together with a quantity of NH₄OH; (c) precipitating the Ni onto the Ti-bearing powder by hydrogen reduction; and (d) washing, filtering and drying the slurry obtained, thereby producing a Ni-coated Ti powder.
 14. The process according to claim 13, wherein the Ni-coated Ti powder manufactured by the process has a Ni:Ti atomic ratio between 0.9 and 1.1.
 15. The process according to claim 13, wherein the Ni-coated Ti powder manufactured by the process has a Ni:Ti atomic ratio between 0.96 and 1.04.
 16. The process according to claim 13, wherein a quantity of ammonium salts is introduced into the autoclave together with the Ti-bearing powder, the Ni salt bearing solution, and the quantity of NH₄OH.
 17. The process according to claim 13, whereby the Ni is precipitated onto the Ti-bearing powder at a temperature of at least 100° C. and a hydrogen pressure in the autoclave of at least 1.4 MPa.
 18. A process of manufacturing a powder mixture comprising a Ni-coated Ti powder, comprising the steps of: (a) providing a Ti-bearing powder and a Ni salt bearing aqueous solution; (b) feeding the Ti-bearing powder and the Ni salt bearing aqueous solution into an (c) precipitating the Ni onto the Ti-bearing powder by hydrogen reduction; (d) washing, filtering and drying the slurry obtained, thereby producing the Ni-coated Ti powder; and (e) intimately mixing the Ni-coated Ti powder with one or both of Ni-bearing and Ti-bearing powder.
 19. The process according to claim 18, wherein the powder mixture manufactured by the process has a Ni:Ti atomic ratio between 0.9 and 1.1.
 20. The process according to claim 18, wherein the powder mixture manufactured by the process has a Ni:Ti atomic ratio between 0.99 and 1.01.
 21. The process according to claim 18, wherein a quantity of ammonium salts is introduced into the autoclave together with the Ti-bearing powder, the Ni salt bearing solution, and the quantity of NH₄OH.
 22. A process of manufacturing a porous sintered body based on a Ni—Ti alloy, comprising the steps of claim 13, and further comprising the step of subjecting the Ni-coated Ti powder to a self-propagating high temperature synthesis operation.
 23. A process of manufacturing a porous sintered body based on a Ni—Ti alloy, comprising the steps of claim 18, and further comprising the step of subjecting the powder mixture to a self-propagating high temperature synthesis operation. 